1. Field of the Invention
The present invention relates to developer compositions for high contrast diffusion transfer photographic materials and to a developing process using the same. In more detail, the present invention relates to developer compositions for producing high contrast images by a diffusion transfer process.
2. Description of the Prior Art
In general, in the graphic arts field high contrast photosensitive materials are used in order to photographically produce line images and half-tone images (half-tone dot images). For example, a half-tone image is produced by printing an original having a continuous tone onto a litho-type photographic sensitive material through a screen and developing using a developer called an infectious developer (see "Photographic Processing Chemistry" pages 163-165, issued by Focal Press Co., 1966).
The operation of an infectious developer can be illustrated as follows. When a litho-type photosensitive material is developed using a so-called litho-type developer solution consisting of, as the main ingredients, hydroquinone and formaldehyde-bisulfite, which is a buffer agent for decreasing the amount of bisulfite ion, the hydroquinone reduces silver halide particles as the development proceeds, and as a result the hydroquinone itself changes to semi-quinone. The thus changed semi-quinone is chemically more unstable than the hydroquinone and has a stronger reductivity, and thus the development is extremely accelerated due to the existence of the semi-quinone. Accordingly, as the development proceeds, the amount of the semi-quinone further increases and, as the result thereof, the development further proceeds. Thus, this kind of infectious developer solution can impart an extremely high gamma-value of about 8-15. By using such a developer solution, the development and the blackening are successively carried out, and this kind of development is called an infectious development.
However, since the process for developing a litho type sensitive material is a negative-negative process, it is not possible to produce images by a positive-positive process. Accordingly, in the case of producing images by a positive-positive process, for example, in the case of producing positive images from a negative original, printing has been carried out using an intermediate separated negative. Therefore, it would be economically advantageous if line images and half-tone images could be directly produced from an original without the necessity of producing an intermediate negative.